With the emergence of high-speed serial interfaces, high-speed backplane technology is replacing conventional low-speed parallel bus technology. However, high-speed serial interfaces may be susceptible to issues including inter symbol interference (ISI). ISI is the spreading of symbols, which leads to energy from one symbol degrading subsequent symbols. ISI may be caused, for example, by amplitude attenuation, group delay distortion, and so on.
Receivers are traditionally configured with a feed-forward equalizer (FFE) that attempts to undo some of the effects on a signal caused by the channel that carries the signal. For channels with extensive attenuation around the Nyquist frequency, a FFE needs to provide significant high-frequency boosting to improve the signal to noise ratio (SNR). However, such high-frequency boosting generally causes noise power enhancement. Noise power enhancement can be partially mitigated if some portion of the channel equalization is performed at the transmitter. The 10GBASE-KR standard therefore defines a three-tap filter to perform channel equalization at the transmitter. Adaptively determining an acceptable, near-optimum, and/or optimum transmit equalizer setting has conventionally been difficult, non-deterministic, and/or time consuming.
An acceptable, near-optimum, and/or optimum transmit equalizer setting may depend on existing channel conditions and thus may not be pre-determinable. Therefore, transmitters and receivers may need to be configured to adaptively determine the settings for the three-tap filter. Some conventional systems for adaptively determining an optimal transmit equalizer rely on minimum mean square error (MMSE) based adaptation and thus may operate in O(N2) time because the systems consider O(N2) data points. Additionally, these conventional systems may ignore the fact that the transmit equalizer needs to be deconvolved from the FFE output samples before being used in the adaptation equation. Implementing a deconvolution circuit can be difficult and can result in a circuit having stability issues. MMSE based approaches may also suffer because the same error signal is used to adapt the timing recovery loop, the FFE/decision feedback equalizer (DFE) setting, and the transmit equalizer setting, which can cause undesired interaction between the various loops which in turn causes tap values to drift, which can degrade the SNR.
Other conventional systems for adaptively determining acceptable, near-optimal, and/or optimal transmit equalizer settings rely on a simplex-based approach. While this search is based on SNR, such a search may be too complex to implement practically. Also, this type of search assumes that transmitter settings can be controlled exactly, which is not a practical reality. In practice, a receiver may only request that a transmitter either increase or decrease tap values in an equalizer. The transmitter may then choose to change the tap values to lie somewhere within the permissible limits described, for example, in a standard. However, the way in which the tap values are changed may not follow a trajectory that facilitates reducing the size of a search space. Since the tap values cannot be controlled exactly, the search mechanism may be slow to converge to a solution, and if the search mechanism does converge, the convergence may not be to an optimal solution.